J. F. de Haan

Scattering matrices of mineral aerosol particles at 441.6 nm and 632.8 nm

We present measured scattering matrices as functions of the scattering angle in the range 5°–173° and at wavelengths of 441.6 nm and 632.8 nm for seven distinct irregularly shaped mineral aerosol samples with properties representative of mineral aerosols present in the Earths atmosphere. The aerosol samples, i.e., feldspar, red clay, quartz, loess, Pinatubo and Lokon volcanic ash, and Sahara sand, represent a wide variety of particle size (typical diameters between 0.1 and 100 μm) and composition (mainly silicates). We investigate the effects of differences in size and complex refractive index on the light-scattering properties of these irregular particles. In particular, we find that the measured scattering matrix elements when plotted as functions of the scattering angle are confined to rather limited domains. This similarity in scattering behavior justifies the construction of an average aerosol scattering matrix as a function of scattering angle to facilitate, for example, the use of our results for the interpretation of remote sensing data. We show that results of ray-optics calculations, using Gaussian random shapes, are able to describe the experimental data well when taking into account the high irregularity in shape of the aerosols, even when these aerosols are rather small. Using the results of ray-optics calculations, we interpret the differences found between the measured aerosol scattering matrices in terms of differences in complex refractive index and particle size relative to the wavelength. The importance of our results for studies of astronomical objects, such as planets, comets, asteroids, and circumstellar dust shells is discussed.

A fast method for retrieval of cloud parameters using oxygen A band measurements from the Global Ozone Monitoring Experiment

The Global Ozone Monitoring Experiment (GOME) on board the ERS-2 is designed to measure trace gas column densities in the Earths atmosphere. Such retrievals are hindered by the presence of clouds. The most important cloud parameters that are needed to correct trace gas column density retrievals for the disturbing effects of clouds are the (effective) cloud fraction and cloud top pressure. At present, in the operational GOME data processor an effective cloud fraction is derived for each pixel, but cloud top pressure is assumed a priori and is deduced from a climatological database. Here we report an improved cloud retrieval scheme, which simultaneously retrieves the effective cloud fraction and cloud top pressure from GOME data. This algorithm, called Fast Retrieval Scheme for Clouds from the Oxygen A band (FRESCO), makes use of reflectivities as measured by GOME inside and outside the oxygen A band (758–778 nm). For validation, the results of FRESCO are compared to effective cloud fractions and cloud top pressures derived with standard methods from colocated measurements made by the Along Track Scanning Radiometer-2 (ATSR-2). The brightness temperatures of the cloudy pixels as measured by ATSR-2 are related to cloud top pressures using temperature profiles from the European Center for Medium-range Weather Forecasts model. Generally, the results from FRESCO and ATSR-2 agree reasonably well. For the effective cloud fractions the average difference (based on a comparison of 322 points) is 0.04; the standard deviation is 0.09. For the cloud top pressures, only points with an effective cloud fraction larger than 0.1 have been compared. For these 236 points the average difference in cloud top pressure is 65 hPa, and the standard deviation is 92 hPa.

Total ozone from the ozone monitoring instrument (OMI) using the DOAS technique

This paper describes the algorithm for deriving the total column ozone from spectral radiances and irradiances measured by the Ozone Monitoring Instrument (OMI) on the Earth Observing System Aura satellite. The algorithm is based on the differential optical absorption spectroscopy technique. The main characteristics of the algorithm as well as an error analysis are described. The algorithm has been successfully applied to the first available OMI data. First comparisons with ground-based instruments are very encouraging and clearly show the potential of the method.

Experimental determination of scattering matrices of randomly oriented fly ash and clay particles at 442 and 633 nm

We present measurements of the scattering angle distribution of the whole scattering matrix for randomly oriented particles of three mineral samples: fly ash, green clay, and red clay at 442 and 633 nm. Fly ash consists of aggregates of nearly spherical particles while green clay and red clay particles represent irregular compact particles. We compare the measured results for fly ash with an experimentally determined average scattering matrix which is based on measurements for a broad selection of irregular mineral aerosol particles. We find that the scattering matrix of our polydisperse sample of aggregates of nearly spherical particles differs considerably from that of compact particles. In addition, the angular distribution of the elements of the scattering matrix (except F22(θ)/ F11(θ)) for fly ash particles seem to be dominated by the single monomers. The effects of small differences in composition on the scattering behavior have also been studied by comparing our experimental results for green clay particles with those obtained by Volten et al. [2001] for red clay particles at the same wavelengths.

Ice crystal shapes in cirrus clouds derived from POLDER/ADEOS‐1

This paper discusses the retrieval of ice crystal shapes of cirrus clouds on a global scale using observations collected with POLDER-1 (POLarization and Directionality of the Earth Reflectance) onboard the ADEOS-1 platform. The retrieval is based on polarized bidirectional observations made by POLDER. First, normalized polarized radiances are simulated for cirrus clouds composed of ice crystals that differ in shape and are randomly oriented in space. Different values of cloud optical depths, viewing geometries and solar zenith angles are used in the simulations. This sensitivity study shows that the normalized polarized radiance is highly sensitive to the shape of the scatterers for specific viewing geometries, and that it saturates after a few scattering events, which makes it rapidly independent of the optical depth of the cirrus clouds. Next, normalized polarized radiance observations obtained by POLDER have been selected, based on suitable viewing geometries and on the occurrence of thick cirrus clouds composed of particles randomly oriented in space. For various ice crystal shapes these observations are compared with calculated values pertaining to the same geometry, in order to determine the shape that best reproduces the measurements. The method is tested fully for the POLDER data collected on January 12, 1997. Thereafter, it is applied to six periods of 6 days of observations obtained in January, February, March, April, May, and June 1997. This study shows that the particle shape is highly variable with location and season, and that polycrystals and hexagonal columns are dominant at low latitudes, whereas hexagonal plates occur more frequently at high latitudes.

Degree of lineair polarization of light emerging form the cloudless atmosphere in de oxygen A-band

We used radiative transfer calculations and model atmospheres for a theoretical investigation of the behavior of the degree of linear polarization, P, of light emerging from the top or the bottom of the cloudless atmosphere in the wavelength region of the O2 A absorption band, between 755 and 775 nm. Results of P are shown for four model atmospheres and for various albedos of the underlying Lambert surface. One of the model atmospheres contains only molecules, whereas the other atmospheres contain also aerosols. It is shown that when the molecular absorption optical thickness of the atmosphere is much smaller than one, which represents the continuum and the weak absorption lines in the band, the state of polarization of the emerging radiation is mainly determined by low-order scattering in the lowest atmospheric layers and by reflection by the surface. In the strong absorption lines, where the molecular absorption optical thickness is much larger than one, we distinguish the three following situations: (1) P of the reflected light is mainly determined by single scattering by molecules and aerosol in the upper atmospheric layers, (2) for small solar zenith and/or viewing angles, P of the diffusely transmitted light is mainly determined by single scattering, and (3) for other geometries, P of the diffusely transmitted light is predominantly due to second-order scattering, with the first scattering taking place in the upper atmospheric layers and the second in the lower layers. For a given surface albedo, the variation of P across an absorption line thus depends on the scattering properties of the atmospheric particles and on their vertical distribution. The surface albedo and P of light emerging from the atmosphere in the principal plane are shown to be related through a simple formula at each wavelength within the absorption band. It is concluded that high-resolution spectropolarimetry in wavelength regions with strongly varying molecular absorption optical thickness can provide valuable information on aerosol at various altitudes in the atmosphere.

Removal of atmospheric influences on satellite-borne imagery: A radiative transfer approach

Abstract Three atmospheric correction methods for satellite-borne imagery are described and compared. The main problem considered is to determine the reflectance of a small Lambertian target which is located on a uniform Lambertian background. The principal differences between the three methods for solving the problem are that i) in the first method the background reflectance is used, but not the apparent background reflectance (the classical method), ii) just the opposite is done in the second method, and iii) both reflectances are used in the third method, thereby eliminating the often inaccurate path radiances. These correction methods may not only be used for atmospheric correction itself, but also to evaluate the use of approximative radiative transfer calculations in atmospheric correction schemes. As an example, we evaluate the Sobolev Modified Approximate Radiative Transfer (SMART) method employing numerical values obtained with the exact doubling method. It is found that the accuracies of the derived target reflectances depend significantly on the correction method used, the actual target and background reflectances, the optical properties of the atmospheric constituents, and the viewing direction considered. It is shown that the first method yields, generally, poorer results for the target reflectance than the second and third method. It is tentatively concluded that replacement of SMART by a more accurate algorithm significantly improves the derived target reflectance only if the aerosol phase function is known with sufficient accuracy.

Benchmark results for single scattering by spheroids

Abstract The light scattering properties of aerosol particles are often described using Mueller matrices and the Stokes formalism. The Stokes parameters of the incident light are then transformed into those of the scattered light by means of a scattering matrix. It is often advantageous to expand the elements of the scattering matrix in generalized spherical functions, especially if the aerosol scattering properties are to be used in radiative transfer codes for calculating the polarized radiation field in a planetary atmosphere. In this paper, expansion coefficients are presented for: (i) randomly-oriented prolate spheroids with aspect ratio a/b = 4.0, size parameter x = 10.079368, and a refractive index m = 1.55 − 0.01i; (ii) randomly-oriented oriented oblate spheroids with a/b = 1.999987, x = 3.0, andm = 1.53 − 0.006i; (iii) an ensemble of randomly-oriented prolate spheroids with a modified gamma size distribution for their semi-major axes, with a/b = 1.4 and m = 1.50 − 0.008i, representing a model for a volcanic aerosol. In addition, values for scattering and extinction efficiencies, albedos, average projected geometric cross-sections, asymmetry parameters, volumes, and scattering matrix elements (for a limited number of scattering angles) have been tabulated. The numerical results presented here have mostly been calculated with the T-matrix method and were checked in various ways. All results are expected to be accurate to six decimal places and can be used as benchmark results.

Parameterized scattering matrices for small particles in planetary atmospheres.

Abstract Parameterized matrices are discussed that may be used as (single) scattering matrices for interpretations of the brightness and polarization of planetary atmospheres containing randomly oriented small particles. A number of guidelines are developed for the construction of such matrices. These guidelines are based on (i) physical conditions for the elements of a natural scattering matrix, some holding for arbitrary scattering angles and some for the exact forward and backward scattering directions only, as well as (ii) theorems for the asymptotic behavior of coefficients in expansions of the matrix elements in generalized spherical functions of the scattering angle. A set of parameterized matrices is introduced and assessed according to these guidelines. These particular parameterizations are especially useful for scattering by particles that are not large compared to the wavelength, particles in the Rayleigh–Gans domain and for a variety of irregularly shaped particles in the visible part of the spectrum. The use of parameterized matrices as scattering matrices is illustrated by deriving their elements as functions of the scattering angle from simulated measurements of the brightness and polarization of light reflected by plane–parallel atmospheres containing aggregated or spheroidal particles. In both cases, the scattering angle dependences of the original elements are retrieved in fair approximation.

Detecting radiances in the O2 A band using polarization sensitive satellite instruments with application to Global Ozone Monitoring Instrument

In this paper, we present numerical simulations of the radiance and the degree of linear polarization of light reflected by the terrestrial atmosphere in the O2 A absorption band, around 760 nm. Since the O2 A band is often used to derive cloud parameters, we included clouds in our model atmosphere. The simulations show that the polarization of the reflected light changes across the O2 A band, and that this change depends strongly on the spectral resolution of the instrument. The polarization of reflected light induces errors in radiances derived from observations by polarization sensitive instruments. For the Global Ozone Monitoring Experiment (GOME) satellite instrument, which measures radiances with about 0.4-nm spectral resolution in the O2 A band, broadband polarization measurements are used to correct the narrowband radiance observations for the instruments polarization sensitivity. Although such correction schemes significantly improve the accuracy of derived radiances in the continuum, they do not account for changes of the polarization in narrow absorption bands, such as the O2 A band. The main purpose of this paper is to investigate for cloudy atmospheres the errors in the derived radiances due to polarization changes across the O2 A band, both for a polarization sensitive instrument with a high spectral resolution and for a GOME-like resolution. If no correction scheme is used, it is found that for nadir viewing directions, the maximum errors in the absorption band can increase by up to about 20% with decreasing width of the spectral response function when the instruments sensitivity for radiation polarized perpendicularly to the principal plane is twice as large as that for radiation polarized parallel to this plane. If, in this case, a correction scheme based on the broadband value of the polarization is used, the radiance errors can still be up to 18% with a high spectral resolution and of the order of a few percent with a GOME-like resolution.